International Concrete Abstracts Portal

The development and use of geopolymers (GP) considerably increased in the construction industry. This paper assesses the suitability of metakaolin-based GP mortars for masonry plastering works, including their comparison to masonry cement (MC) mortars and compliance to relevant EN 413-1 and ASTM C91 specifications. Three classes of GP mortars prepared with different metakaolin-to-limestone ratios are tested; the sodium hydroxide and sodium silicate activators contained air-entraining molecules to secure approximately 10% ±2% air content. Test results showed that GP mortars exhibited excellent water retention and increased rheological properties, which was related to higher viscosity of alkaline solution that increases stickiness and overall cohesiveness. For given limestone concentration, the mechanical properties of GP mortars including the pulloff bond strength and sorptivity were remarkably better than MC mixtures. Almost 90% of ultimate compressive strength was achieved after 7 days for GP mortars cured at ambient temperature, while this varied from 55 to 80% for MC mixtures cured in moist conditions. This can be particularly advantageous in masonry applications to speed up construction operations while, at the same time, eliminate the hassle of moist curing normally required with cement-based plasters.

This paper presents the stress-strain behavior of structural synthetic fiber-reinforced cellular lightweight concrete (CLC) stack-bonded prisms under axial compression. Masonry compressive strength is typically obtained by testing stack-bonded prisms under compression normal to its bed joint. CLC prisms with cross-sectional dimensions of 200 x 150 mm (7.87 x 5.90 in.) with an overall height of 470 mm (1.54 ft) were cast with and without different dosages of synthetic fiber reinforcement. Polyolefin was used as a structural fiber reinforcement at different volume fractions (vf) of 0.22, 0.33, 0.44, and 0.55% with and without microfiber dosage of 0.02%. Experimental results indicate that the presence of fibers helps in the improvement of strength, stiffness, and ductility of CLC stackbonded prisms under compression. Test results also signify that the hybrid fiber reinforcement provides better crack bridging mechanism both at micro and macro levels when compared to only macrofibers. Simple analytical models were developed for stress-strain behavior of CLC blocks and stack-bonded CLC prisms based on the experimental results with and without fibers under compression.

An experimental investigation was conducted to investigate the effects of replacing varying percentages of fine natural aggregates with crumb rubber in concrete masonry units (CMUs), creating rubberized concrete masonry units (RCMUs). The mechanical and physical characteristics of RCMUs having 0, 10, 20, and 37% crumb rubber were investigated and presented in this paper. The unit weight and water absorption of RCMUs were measured. A scanning electron microscope (SEM) analysis was used to study the global structure for RCMUs and the interfacial zone. RCMUs were also exposed to extreme weather conditions for 72 days inside an environmental chamber. Furthermore, RCMUs were subjected to rapid freezing-and-thawing tests. The RCMUs, as well as grouted and ungrouted masonry prisms, were tested under monotonic and cyclic axial loads. The results indicated that RCMUs with high rubber content displayed higher values of axial ultimate strains. RCMUs exhibited a significant strain softening while, conversely, failure was quite brittle in CMUs. RCMU specimens exhibited an improvement in compressive strength after several cycles of severe weather exposure. The CMU specimens, however, exhibited degradation in their compression strength capacity. The water absorption was higher in RCMUs than it was in the CMU prisms.

A testing program was conducted to determine whether concrete masonry prisms constructed with Type M mortar and grouts containing high volumes of supplemental cementitious materials (SCMs) could meet minimum masonry compressive strength requirements. Research focused on replacing portland cement (PC) with Class F fly ash and ground-granulated blast-furnace slag (GGBFS) in quantities larger than those currently allowed by technical standards. In addition, the research evaluated the development of the compressive strength of the prisms with time. Thus, specimens were tested at 14, 28, 42, 56, and 90 days. The control prism group contained grout with only PC. In the second prism group, the grout had Class F fly ash replacing PC at rates of 45, 55, and 65% while in the third prism group the grout had Class F fly ash and GGBFS combinations replacing PC at rates of 65, 75, and 85%. The compressive strength of all prisms exceeded the minimum compressive strength requirement of 10.34 MPa (1500 psi) at 28 days, although the 65% fly ash grout mixture itself did not meet the minimum grout compressive strength of 13.79 MPa (2000 psi) at 28 days. A lower estimate of the ultimate strength of grouted prisms constructed with grouts containing high volumes of SCM can be estimated by multiplying the strength measured at 14 days by 1.2 and 1.3 for prisms with binary and ternary grouts, respectively.

Early-age carbonation curing of concrete products results in improved strength, increased surface hardness, and reduced surface permeability to water, as well as the reduction of efflorescence. Carbonation reactions between carbon dioxide and calcium compounds result in permanent fixture of the carbon dioxide in thermodynamic stable calcium carbonate. The moisture content, relative humidity, and temperature profile of the hydrated system have considerable and important influence on the rate and ultimate extent of carbonation. During carbonation, CO2 penetrates the surface of concrete and reacts with cement hydration products—namely, calcium hydroxide and calcium silicate hydrates—to form carbonates. This study quantifies carbon sequestration levels in concrete masonry units using various curing methodologies. The test results of a dynamic pressurized CO2 curing chamber and normal ambient CO2 pressure at various concentrations levels are compared to traditional kiln curing procedures. Early compressive strength profiles for 30% CO2 cured concrete masonry units (CMUs) are equivalent to 100% CO2 cured CMUs and exceed the traditional kiln-cured compressive strengths. Carbon sequestration reduced water requirements by 20% for optimum strength performance and provided water conservation opportunities.